The present invention relates to a method of forming a web with apertures. More particularly, the invention relates to a method of fabricating a shadow mask for use in a color cathode ray tube.
Cathode ray tubes are used in a variety of devices such as televisions and computer displays. The most important element in obtaining a high resolution cathode ray tube is producing a precisely etched shadow mask. It is generally recognized that a two-step etching process is needed for the successful manufacture of a uniformly etched, high resolution shadow mask. Two-step etching helps avoid the rapid and uncontrolled dimensional changes that occur when etched cavities intersect during simultaneous etching of both major surfaces during one-step etching.
The references that describe two-step etching indicate that the cavity formed in the first etching step should be completely filled before performing the second etching step. One such two-step method of fabricating a shadow mask is discussed in Kubo et al. U.S. Pat. No. 3,679,500. First, both sides of a web are coated with an etch resistant material. After the etch resistant material is patterned, a first side of the web is then etched to form a first cavity therein. Next, the first cavity is filled with an etch resistant material. Finally, the web is etched from a second side, opposite the first side, to form a second cavity that communicates with the first cavity to form an aperture.
Another two-step method of producing a shadow mask is described in Ohtake et al. U.S. Pat. No. 4,689,114. A web is oriented with a first surface up and the first surface of the web is then etched to form a cavity. Next, the web is flipped so that the first surface is now facing downward. An etch resistant layer is applied to the first surface and the second surface is etched until a cavity etched in the first surface communicates with the cavity etched in the first surface at a constricted portion. The method described in the Ohtake et al '114 patent attempts to completely fill the cavity in the first surface with the etch resistant layer.
Another attempt to increase the quality of shadow masks produced with a two-step etching process is disclosed in Toei et al. U.S. Pat. No. 5,200,025. The Toei et al. patent describes spraying a high pressure fluid at the web after the first etching step. The fluid removes the etch resistant material that overhangs a cavity etched during the first etching step. Then it is possible to more completely fill the cavity with etch resistant material before conducting the second etching step.
Rounding of the edges defining the cavity at the constriction point has been considered important in providing a shadow mask with excellent qualities. While one-step etching provides internal edge rounding, the overall qualities of a shadow mask produced using one-step etching has been deemed to be inferior to a shadow mask manufactured using a two-step etching process. One reference that addresses the need to achieve internal edge rounding in a multi-step etching process is Ohtake et al. U.S. Pat. No. 4,662,984. The method in the Ohtake et al. '984 patent is similar to the method in the Ohtake et al. '114 patent. However, after stripping the etch resistant material from the etched web, this method includes an additional etching step. In this step, etching solution is sprayed at both sides of the web to round the edges defining the cavities at the constriction point.